1. Field of the Invention
The invention concerns a method for the indirect determination of local irradiance in an optical system, in particular for a partial region of an optical system, such as the beam profile at a selected site of the beam path or of the irradiance on a component of the optical system, wherein the method is applied, in particular, to an optical system with an EUV light source as an illumination source.
2. Description of the Related Art
Optical systems with EUV illumination sources are developed, in particular, for EUV lithography systems, for the purpose of obtaining, by use of wavelengths of ≦193 nm, pattern widths for electronic components in the submicron range. For this lithography technique with soft x-rays, so-called EUV lithography is preferred, the wavelength range being from λ=11 nm to 14 nm and, in particular, λ=13.5 nm, whereby the use of numeric apertures in the range of 0.2-0.3 is discussed. For example, synchrotron sources or plasma sources can be used as illumination sources for this wavelength region.
The image quality in EUV lithography is determined, on the one hand, by the projection objective, and, on the other hand, by the illumination system. The illumination system will provide an illumination that is as uniform as possible of the field plane, in which the pattern-bearing mask, the so-called reticle, is disposed. The projection objective images the field plane in an image plane, the so-called wafer plane, in which a light-sensitive object is disposed. Projection exposure systems for EUV lithography are designed with reflective optical elements. The shape of the field of an EUV projection exposure system is typically that of an annular field with a high aspect ratio of 2 mm (length of the scanning slit)×22-26 mm (width of the scanning slit). The projection systems are usually operated in scanning mode, whereby the reticle will be moved in the field plane and the light-sensitive object, typically a wafer with a suitable photoresist, will be moved synchronously in the image plane, relative to one another. With respect to EUV projection exposure systems, reference is made to the following publications:    W. Ulrich, S. Beiersdörfer, H. J. Mann, “Trends in Optical Design of Projection Lenses for UV-Lithography and EUV-Lithography” in Soft-X-Ray and EUV Imaging Systems, W. M. Kaiser, R. H. Stulen (Editors), Proceedings of SPIE, Vol. 4146 (2000), pages 13-24 and    M. Antoni, W. Singer, J. Schultz, J. Wangler, I. Escudero-Sanz, B. Kruizing a, “Illumination Optics Design for EUV-Lithography” in Soft X-Ray and EUV Imaging Systems, W. M. Kaiser, R. H. Stulen (Editors), Proceedings of SPIE, Vol. 4146 (2000), pages 25-34.
A problem that occurs particularly for optical systems in the EUV range is that beam profile measurements for obtaining and also for monitoring the state of adjustment of the optical components of the optical system are difficult to conduct due to the short wavelengths. Information of the beam profile is also of advantage for evaluating the performance capacity of optical components.
This same problem arises also in determining the irradiance on the optical components used in the illumination system. This determination is of particular importance for EUV illumination systems, since here reflective optical components are used exclusively, which can be constructed as grazing-incidence or normal-incidence systems. Characteristic of such EUV optics is their limited reflectivity, which deteriorates in operation, due to deposits or possible degradation defects on the mirror surfaces. This problem again makes it necessary to regularly examine the local irradiance, at least in partial regions or on selected subcomponents of such an illumination system, whereby, in particular, in addition to the characteristic of spatial irradiance, a quantitative determination or a sufficiently accurate estimation of the irradiance is also required.
The methods that have become known from the prior art for monitoring optical systems in the EUV region concern EUV light sources almost exclusively. Thus, in US 2003/0146391, a detector is proposed for monitoring the irradiated light power of an EUV plasma source, which is found in a detection beam path separate from the illumination beam path. Here, the étendue value of the detection beam path is adapted to any illumination beam path in order to simulate this as precisely as possible. It is a disadvantage in US 2003/0146391, however, that the measurement of the irradiated light power in a separate beam path is insufficient by itself to assure that an error with an effect on the imaging in the image plane does not occur, namely an inhomogeneously illuminated field, a telecentric error, or a dose error.
The determination of secondary electrons, which come from an absorbing mask for proximity exposure with x-ray light, has become known from JP 63-072,116. A locally-resolved measurement for the determination of dose errors or a contamination of optical components, however, is not possible.
A method for monitoring the degree of fouling of a mirror for synchrotron radiation has become known from JP 05-288,696. Here, the photocurrent integrated over the mirror surface is determined, but there is no information on the local distribution of irradiance.
Measurements of the beam profile, which are the subject of the present application, are usually conducted with semiconductor detectors, in particular, in the visible or infrared regions of the spectrum. A typical field of application is the measurement of laser beam profiles or the determination of illumination characteristics of illumination systems. Usual here is the use of silicon, germanium or gallium arsenide detectors, which are comprised of a combination of a linear detector with a pin diaphragm and a precise positioning system, which can be used for the scanning measurement of a beam profile. Alternatively, area detectors, such as CCD sensors or CMOS sensors, which are usually integrated in a camera system, are used for this purpose. The advantage of area detectors when compared with linear detectors, above all, is the savings of time when conducting a measurement.
With respect to beam and illumination diagnostics for illumination systems which use short wavelengths, in particular in the EUV region, a fluorescence converter can be utilized, with which radiation in a wavelength region of 10 nm up to approximately 350 nm can be successfully converted into the visible wavelength region, so that a standard camera with a Si—CCD detector can be used for taking images. It is a disadvantage with the use of fluorescence converters, however, that they are suitable only to a limited extent for beam profile measurements for illumination systems with a high illumination intensity. This applies, in particular, to the EUV region in which the absorbed radiation rapidly leads to an overheating and to the degradation of the fluorescence converter or to a change of the conversion efficiency and thus leads to a falsification of the measurement. This limitation essentially applies also to the DUV and the VUV wavelength regions, in addition to the EUV wavelength region. Thus, EUV photodiodes in row or matrix arrangement, which measure the photocurrent or photoelectrons in a locally-resolved manner, are preferred for the EUV region.
Additional requirements for a method or a measurement system for determining the local irradiance in an optical system for the wavelength region of ≦193 nm and, in particular, in the EUV region, result from the requirements relative to constructability and vacuum conditions. The known detector systems often can be integrated into the illumination beam path and encapsulated by vacuum technology only at increased expense. Furthermore the problem occurs that such detector systems necessarily interrupt the beam path during the measurement and, therefore, further measures are necessary for positioning of all measurement components used for the measurement of the beam profile or the illumination characteristic. Continuous measurements or routine inspections, which can be conducted without interfering with the state of adjustment, are thus possible only with difficulty with the known detection systems.
Another disadvantage with the use of the known measurement systems for determining beam profiles or the local irradiance on an optical component results from the fact that detectors are adapted to the wavelength region that is used in each case, for example, by means of a coating layer using a film filter, so that common detection systems are not suitable for broadband spectra—IR, VIS, UV, DUV, VUV and EUV. A universal measurement method or measurement system that can be used over a broad wavelength region is of advantage in itself. For applications in EUV lithography systems, in particular, an adjustment can then be carried out with adjustment illumination outside the used wavelengths for EUV, without the need for changing the measurement system. The same set of problems is also to be encountered in optical systems for the DUV and VUV wavelength regions.